1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a color liquid crystal display device. The invention relates also to a method for producing the liquid crystal display device.
2. Description of the Related Art
Recently, applications of a liquid crystal display device to a word processor, a note-type personal computer, a portable television receiver called a pocket TV, or the like have rapidly been increased.
In a liquid crystal display device, conventionally, a TN (twisted nematic) mode, or an STN (super twisted nematic) mode has been used. When the former, or the TN mode is employed, a liquid crystal display element is disposed between one set of polarizers, and a monochromatic display is conducted by using optical characteristics of the liquid crystal display element, i.e., the optical rotary characteristics appearing when no voltage is applied, and the polarization dissolution characteristics appearing when a voltage is applied.
In contrast, the latter, or the STN mode is a liquid crystal display structure which is similar to the TN mode and widely used in a display device for a word processor. In the STN mode, the twist angle of a liquid crystal layer of the liquid crystal display is set between 180 to 270 deg. The STN mode is characterized in that the twist angle of the liquid crystal layer is increased by 90 deg. or more and the angle of polarization axes of the polarizers are optimized, so that a sudden change in molecular orientation due to a voltage application influences the change in birefringence of the liquid crystal layer, thereby realizing electro-optical characteristics having a sharp threshold. Accordingly, the STN mode is suitable for a simple matrix driving system. On the other hand, the STN mode has a drawback that the background of a display is colored in yellow-green or dark blue because of the birefringence of the liquid crystal. In order to eliminate this drawback, a liquid crystal display element has been proposed in which color compensation is made by stacking an STN display panel with an optical phase compensation plate, or a phase difference panel made of a polymer such as polycarbonate, thereby attaining a monochromatic display. At present, a liquid crystal display element of this structure is commercially available as a so-called paper white LCD.
When a color display is to be done, microcolor filters of, for example, red, blue and green are disposed for each pixel of a liquid crystal display element of the TN mode which can conduct a monochromatic display as described above, or that of the STN mode in which color compensation is made, and a multicolor or full color display is conducted in accordance with the additive color mixing by using the optical switching characteristics. At present, a liquid crystal display device which can conduct such a color display is used as a display device for a portable apparatus such as a so-called liquid crystal television receiver or a laptop computer in which an active-matrix driving system or a simple matrix driving system is employed.
As a coloring system for a liquid crystal display device of the STN mode, a multicolor display method in which a multicolor display is conducted by applying a voltage to a compensation layer of Double Layer Super Twisted Nematic (DSTN) to control compensation conditions as been proposed (C. Iijima et al., JAPAN DISPLAY '89, p. 300).
Other color display methods are those according to the interference color method and including a so-called ECB (electrically controlled birefringence) mode, a DAP (deformation of vertical aligned phase) mode, and a HAN (hybrid-aligned nematic) mode. These modes use the birefringence of a liquid crystal molecule. A liquid crystal display device of such a mode is so structured that polarizers are respectively disposed on the both sides of a liquid crystal cell having a liquid crystal layer in which nematic liquid crystal molecules are aligned in a specified direction with respect to a substrate. When a voltage is applied to the liquid crystal layer, the initial orientation of the molecules is changed to cause a change in anisotropy of refractive index, and therefore the color of light passing through the liquid crystal layer is changed to conduct a color display.
The display modes of TN, STN, ECB, DAP and HAN are well known techniques and their operation principles are described in the "HANDBOOK OF LIQUID CRYSTAL DEVICE", the 142nd committee of Japan Society for the Promotion of Science, 1989, pp. 329-352.
When a color display is to be conducted in a liquid crystal display device of the TN mode or the STN mode, it is required to use a color filter as described above. This causes a problem as follows. When color filters of three colors are used, for example, at least 2/3 of light of the visible region is absorbed, and about half of the light is absorbed by polarizers which are disposed in the vicinity of the liquid crystal panel, resulting in that the total reflectance of the whole of the panel is reduced to 15% or less, whereby causing a problem that the display is very dark.
In a color liquid crystal display device such as ECB, or DAP using the interference color, colors to be shown can be changed depending on an applied voltage. However, the range of an applied voltage is very narrow so that even a small variation in the applied voltage causes the hue to be changed. Furthermore, the number of colors which can be used in the display is limited.